1. Field of the Invention
The present invention relates to an image forming apparatus that uses a rotating polygon mirror having a plurality of reflection planes to deflect laser light with which a photoreceptor is exposed.
2. Description of the Related Art
Multi-lasers, which emit a plurality of laser beams have come to be used more and more often as image forming apparatus, are increasingly enhanced in speed and image quality. The enhancement of image forming apparatus in image quality is accomplished by raising image resolution to 1,200 dots per inch (dpi) or 2,400 dpi, raising gray scale gradient through the refining of multi-level expression, or the like.
On the other hand, raising resolution or gray scale gradient to enhance image quality makes band-like streaks, density unevenness, and the like in the sub-scanning direction, namely, banding, conspicuous. Banding is caused by, in addition to conveyance irregularities of a photoreceptor, a conveyer belt, or other components in a conveyance driving system, exposure unevenness due to a positional deviation in the sub-scanning direction as a result of an axial error in which the rotation axis of a polygon mirror in an exposure unit changes its position, or an optical facet angle error in which the tilt of a reflection plane of a polygon mirror with respect to the rotation axis of the polygon mirror changes. For instance, an axial error or optical facet angle error of a polygon mirror causes a positional deviation from an ideal spot in the sub-scanning direction for each reflection plane of the polygon mirror, and causes density unevenness in a cycle determined by the number of reflection planes of the polygon mirror.
As a solution to this, Japanese Patent Application Laid-open No. 2008-116664 has proposed a technology for an image forming apparatus in which image quality deterioration due to an optical facet angle error of a rotating polygon mirror is corrected by controlling the laser light amount. In this technology, each reflection plane of a polygon mirror is measured for the amount of an optical facet angle error at the time the image forming apparatus is assembled in a factory, and laser light amount correction data determined by the amount of the optical facet angle error is stored in a memory. The image forming apparatus identifies which reflection plane, out of the plurality of reflection planes of the polygon mirror, laser light enters, reads light amount correction data that is associated with the identified reflection plane out of the memory, and corrects the laser light amount based on the read correction data. Light amount correction data is generated to even out the sparseness/denseness of laser light scanning lines which is caused by an optical facet angle error. Specifically, when the scanning line interval from one reflection plane to another reflection plane is sparser than a given interval (resolution) due to an optical facet angle error, the laser light amount of at least one of the sparse scanning lines is increased and, when the scanning line interval from one reflection plane to another reflection plane is denser than the given interval due to an optical facet angle error, the laser light amount of at least one of the dense scanning lines is decreased. Adjusting the laser light amount in this manner makes density unevenness caused by an optical facet angle error less conspicuous. The amount of an optical facet angle error is measured with a high-precision measurement jig at the time a laser scanner is produced in mass, and light amount correction data generated based on the result of the measurement is recorded in a read only memory (ROM) or the like of the laser scanner. This light amount correction data is also used to correct the light amount when an image is actually formed.
However, the extent of an axial error, optical facet angle error, or the like of a polygon mirror can change after measurement due to an impact from an unforeseen accident after the product is shipped, or from the influence of temperature, humidity, and vibrations in the place where the product is installed. The extent can change also because of the aging of a motor shaft after shipment. In such cases, information measured and recorded in a ROM or the like in advance becomes useless and an effective correction cannot be made.
If banding worsens, the correction made ends up being an over-correction, or an appropriate correction can no longer be made, the laser scanner itself needs to be replaced. The resultant problem is that the cost of replacement parts and replacement work mounts.
The present invention has been made in view of the problems of the related art described above, and a main object of the present invention is therefore to provide an image forming apparatus configured to effectively correct, with a simple configuration, banding such as density unevenness due to an optical facet angle error or the like of a polygon mirror.